Yes you may.
I have already started to list what we would use and how. Please add to it at
http://docs.codehaus.org/display/XPR/Migration+to+JDK1.5.
I am planning on cleaning up the core object model and using full support for 1.5 at that time.
Jacques
I suggest you don't spend a lot of time converting what already works and is tested, but as you refactor and add new code, by all means start using the new JDK1.5 features.
Let me know if you want some help with it.